The need to store water in excavated reservoirs, or to prevent water seepage through earth dams has, since ancient times, exercised the human mind with a view to seeking ways for making soil impervious. One of the earliest methods of preventing leakage consisted in covering the porous ground with brickwork, stone masonry, or an impervious clay cover. These thick and heavy covers became too expensive with increasing cost of labor, and new ways were looked for. It is modern practice to apply a relatively thin, continuous layer of an impervious material to the sides and bottoms of the structures concerned (e.g. see U.S. Pat. No. 3,309,875), either by spraying substances capable of setting (e.g. see U.S. Pat. No. 3,585,804) or by stretching strips of thin sheeting and joining them along their edges. Another way is to make the soil itself impermeable by compacting it--provided that it lends itself to this process--or by chemically treating its upper layer.
In order to prevent physical damage to the comparably thin, impermeating layer it is common practice to protect the impervious material forming it, by covering it with an additional layer of soil; but in spite of the protection, small holes or tears are inevitable: they are the results of such haphazard causes as careless treatment, insufficient sealing along the seams, natural earth movement, growth of roots and others.
Even small holes may eventually result in considerable seepage, since the flow through a hole in a thin membrane represents flow that would result without a membrane altogether but through an area larger than that of the hole. In other words, small holes reduce the resistance to flow much more than may be expected having regard to their area.
Furthermore, covering a large area with impermeable strips of sheeting requires very careful laying and the aligning of their edges subsequently to being joined, and the cost of the entire operation is considerably increased by the final earth cover that must be placed over the sheeting as protection against physical damage, rendering the impermeating of a reservoir very expensive, even if low-priced sheet material is used. One attempt to lay a sub-soil impermeable strip is shown in U.S. Pat. No. 3,590,588.